Granulocytes and moncytes are derived from a small population of bone marrow progenitor cells which can be assayed in vitro by their ability to form colonies of myeloid cells in semisolid medium (colonyforming unit, CFUGM). This colony formation requires the presence of specific glyocoproteins termed colony stimulating factors (CSF), produced by activated T cells and monocytes. CFUGM are present in low frequency in bone marrow, and it has been very difficult to isolate these cells. As a result, very little is known about the events which regulate proliferation and differentiation of these critical cells. The longterm objectives of this project are to understand the mechanisms which regulate CFUGM. An important step in this process is the development of techniques to purify these rare cells. Using a series of monoclonal antibodies developed in our laboratory, we have developed a technique to enrich CFUGM approximately 100 fold from normal bone marrow. These purified CFUGM have been used to: 1) develop a sensitive and specific assay for CSF activity, 2) immunize mice to produce new progenitor cell antibodies, 3) directly investigate the role of HLADR surface antigens in the regulation of myelopoiesis, and 4) investigate the effects of cloned lines of human natural killer cells on CFUGM. In the continuation of this project, it is proposed to use purified progenitor cells to assist in the screening for monoclonal antibodies to the major growth factor of CFU-GM, G,-CSF; to generate monoclonal antibodies to the GMCSF receptor using an antiidiotype approach, and to initiate investigation of the early transmembrane events (calcium flux, membrane potential changes) induced in immature myeloid cells by GM-CSF and a variety of other humoral regulators of myelopoiesis. It is anticipated that an improved knowledge of the mechanisms of regulation of normal stem cells may also be ultimately useful in understanding the abnormal regulation of myelopoiesis in disorders such as AML.